1. Field of the Invention
The present invention relates to a mobile object position estimation apparatus mounted aboard a mobile object and estimation method. Specifically, it relates to a mobile object position estimation apparatus for estimating the position of a mobile object with high precision, free from the influence of the road contour and the surrounding environment.
2. Description of Related Art
A conventional mobile object position estimation apparatus is described in Japanese Patent Application Publication 9-189564 (JP 9-189564A). The mobile object position estimation apparatus is mounted in a vehicle for example and estimates the position of the mobile object. This type of apparatus is typically included within a navigation system.
Typically, a Global Positioning System (GPS) is used for detecting and estimating the position of a mobile object so as to acquire the absolute coordinates of the mobile object.
To accommodate situations in which the condition of reception of the radio signal from the GPS satellite is poor, a group of inertial navigation sensors, such as acceleration and yaw rate sensors, are provided so that the position of the mobile object can be estimated based on the outputs from the group of inertial navigation sensors.
In most conventional GPS, the measurement accuracy is poorer than that of real-time kinematic GPS (RTK-GPS), and an error of approximately several tens of meters can occur. The accuracy of the inertial navigation sensor devices characteristically worsens with so-called drift as time elapses.
Given this situation, if the detected and estimated position of the mobile object in a navigation system is discovered to be off a road when a check is made against map information previously held by the mobile object, so-called map matching processing (hereinafter “map matching”) is performed so as to correct the position of the mobile object to bring the mobile object to some position on the road before indicating the position to the user.
In JP 9-189564A, an apparatus that corrects the inertial navigation sensor devices error and output error based on map information when the position and the speed of the mobile object are calculated using the inertial navigation sensors because of a failure to receive the GPS signal is described.
However, both map matching and the correction processing using map information described in JP 9-189564A assume that the map information is accurate, and constitute processing that brings the accuracy of the detected and estimated mobile object position in line with the accuracy of the map information. The accuracy of the mobile object position after correction, therefore, does not exceed the accuracy of the map information.
Map information generally consists of a plurality of nodes and straight line links connected between the nodes. The map matching process corrects the offset in the detected and estimated mobile object position to coincide with some position on a link when the mobile object position results is not positioned over a link.
Because the capacity of the memory media used to store map information is finite, however, rather than storing as map information of all the points of inflection of the road contour at which nodes should be set, there is some “skipping” of nodes. Thus, the road contour described by the nodes and links included in the map information is not necessarily a complete representation of the actual road contour.
In the map matching and in the correction process described in JP 9-189564A, which use map information (node and link information) the accuracy of which is not necessarily good as correct information serving as a benchmark, even after correction the mobile object position could still have a relatively large error. Additional inaccuracy may be introduced into the detected and estimated position of the mobile object by map matching and other interpolation processes or the like.
An example is shown in FIG. 1A and FIG. 1B. As shown in FIG. 1A, node N1 and node N3 are set in the map information corresponding to the actual road contour R. Because of, for example, a limitation in the capacity of the storage media, even though node N2, shown by broken lines, is at a curve in the road, node N2 is not included in the map information. This being the case, the links L1 and L2, which are also shown by broken lines, are similarly not included in the map information, and the resulting map information reflects the nodes N1 and N3 as being connected by the link L3. Thus, according to this map information the road contour is perceived and displayed as running along the link L3.
If map matching is performed using such map information as a benchmark, even if the position of the mobile object detected and estimated by GPS or the inertial navigation sensor devices is P1 or P2 which is the actual position on the road, as shown in FIG. 1B, map matching results in the erroneous correction of the mobile object position to the position V on the link L3.
Various methods have been proposed for improving the accuracy of map matching. However, in map matching, which has as its characteristic object to correct as noted above, the detected and estimated position of the mobile object to a point on a link that is included in the map information used, regardless of the details of the specific method used, there is an intrinsic limitation of the accuracy of position finding to the accuracy of the map information as an asymptote.
Even if the detected and estimated position of the mobile object is corrected so that there is an apparent solution to the display problem in the navigation system, there are still various problems if the mobile object position accuracy is poor.
For example, in a so-called navigation-coordinated system for driving support (for example, in stopping assistance control by an alarm or by braking intervention at an intersection), a high-accuracy GPS apparatus is mounted aboard a vehicle, and high-accuracy position detection is performed with an error not exceeding several centimeters. In this system, the detected vehicle position is used in various control applications and, even if the receiving condition of the GPS signal is not poor, it is necessary to continue to detect the position of the vehicle in order to continue vehicle control. In such cases, as described in JP 9-189564A, in a method for correcting the inertial navigation sensor devices and the outputs thereof using map information, deterioration of the detected and estimated position accuracy could be so serious as to make it difficult to continue control of the vehicle based on vehicle position.